Wireless communication needs worldwide have increased dramatically in the last few years, particularly in the amount of devices being used and the demand for higher data rates. The amount of available radiofrequency (RF) spectrum is limited, and the result has been an incredible increase in the licensing costs for specific frequency bands. The limited frequency spectrum has prompted telecommunication equipment manufacturers to either increase the spectral efficiency by using more complex modulation schemes or to utilize available bandwidth at higher RF frequencies.
The highest data rate that a RF communication system can handle is generally determined by the type of modulation scheme used by the communication system. One of the most spectrally-efficient modulation schemes currently available is Quadrature Amplitude Modulation (QAM). In a QAM system, there are two carrier waves, each having the same frequency, but differing phase by ninety degrees (90°). One of the carrier waves is termed the “I” or “In-Phase” signal, and the other is termed the “Q” or “Quadrature-Phase” signal. The I and Q carrier waves are typically (re)generated by a voltage controlled oscillator (VCO) circuit. The use of QAM as a modulation scheme permits significantly more information to be carried by a particular bandwidth than modulation schemes such as Amplitude Modulation (AM) and Phase Modulation (PM).
There are many parameters in a RF system that can limit the order of QAM used. One of the main parameters is phase noise. A high proportion of phase noise is generated by local oscillators (LOs) used in frequency conversion in a RF system. Many local oscillators use a VCO within a phase locked loop (PLL) to generate a highly stable local oscillation with low phase noise.
The phase noise in a VCO output can result in cross-talk and increased bit error rates in both wired and wireless communications system. Recently, as switching speeds of Complementary Metal Oxide Semiconductor (CMOS) processes have increased, the design of VCOs in CMOS is attractive because manufacturing is less expensive and designing the VCO in CMOS allows greater integration with other CMOS digital circuitry.
The full integration of transceivers on chips implies the use of low intermediate frequency (IF) or zero IF architectures that require quadrature signals for In-phase/Quadrature-phase (I/Q) modulation and demodulation and image rejection.
Several techniques exist to generate the “quadrature” I and Q signals required for QAM. For example, a VCO running at the “double” frequency can be divided by two to give quadrature. This solution shows poor quadrature accuracy, as it requires an accurate 50% duty cycle VCO. A VCO followed by a polyphase filter gives quadrature, however, it requires buffers that increase the power consumption considerably. Alternatively, two separate VCOs can be forced to run in quadrature by using coupling transistors. However, this approach suffers from a trade-off between accurate quadrature and low phase noise. Moreover, the coupling transistors increase the power consumption. To circumvent the phase noise penalty, additional 90° phase shifters can be placed at the gates of the coupling transistors. However, the increase in power consumption remains.
Recently, an alternative quadrature topology has been proposed, where negative resistance transistors are cascoded by the coupling transistors. Andreani, P., “A Low-Phase-Noise Low-Phase-Error 1.8 GHz Quadrature CMOS VCO”, ISSCC Dig. of Tech. Papers, pp. 290-291, February 2002. Although this approach gives low phase noise and does not increase power consumption, the technique is not well suited for implementation of widely tunable oscillators in the 5 GHz range. This is because the coupling transistors have to be about five (5) times larger than the negative resistance transistors, thus loading the oscillator with large parasitic capacitors that limit the tuning range.
To summarize, the solutions that presently exist for generating quadrature suffer from an increase in phase noise and/or an increase in power consumption, or they result in a limited tuning range, when used at high frequencies of oscillation.
Thus, there is presently a need for a VCO that generates quadrature over a wide tuning range without suffering from an increase in power consumption and phase noise.